Mechanical presses are commonly used to form industrial products such as automobile parts which are stamped or pressed from steel blanks or workpieces. Today's large mechanical presses are traditionally and most often driven by a flywheel. The function of the flywheel is to store the necessary energy to carry out a pressing, stamping, punching etc operation. A motor drives the flywheel so that before the start of a press operation the flywheel is rotating at the speed at which the pressing will occur. Thus the flywheel motor has the function of driving a high inertia load at a substantially constant speed.
In such presses, parts are pressed between an upper and a lower die. The upper die is connected to the press slide, which moves up and down in the slide guides, while the lower die is either fixed or mounted on a bed. The slide motion is driven by the press mechanism, which is located in the upper part of the press, known as the crown. The press mechanism consists of speed-reducing gears and a mechanism which translates rotating motion of the gears into linear motion of the slide. This translation can either be a relatively simple eccentric mechanism, or a more complicated link-drive mechanism. The gears today are driven by the flywheel, which is connected to the so-called main shaft (or high-speed shaft) through a clutch. A brake is also connected to this same shaft.
In a conventional mechanical press the press continues to rotate after each pressing stage is completed until its eccentric wheel has rotated one complete turn. During this second stage following pressing, the motor driving the flywheel will slowly increase the rotational speed and regain the normal pressing speed. At the end of the operation, the clutch is disengaged and a brake is used to stop the motion of the press. In the traditional mechanical solution, press speed is fixed and proportional to flywheel speed during the complete operation. Thus, if pressing has to be done at a low speed (for quality or other technical reasons), the complete operation will occur at low speed. This results in a long cycle time, and therefore, a low production rate. To address the problem of low speeds in the non pressing stage of a press production cycle presses with variable speed drive motors, known as servo presses, or hybrid servo presses, have been developed. For example, US2004/003729, entitled Drive unit and drive method for press, assigned to Komatsu, describes a press drive unit with a first drive system for driving a flywheel with a main motor and a second drive system for driving the drive shaft at variable speed with a sub motor.
To provide a servo press, one option is designing completely new press mechanics, and integrating a servo motor and associated transmission into this new design. This option, a new press design, is the option which can give a design which is best suited for servo operation, as the design can be optimized. For example it can be designed for optimal controllability of the press during the pressing phase, or for highest possible productivity. However, this option has high risks for both press manufacturers and their customers: the design will be new and thus unproven, and most often manufacturers and customers have, as yet, few or no clear specifications for how such a design should perform. As a result, different manufacturers will likely offer very different servo press designs, some slower than existing mechanical presses, some with extremely high power requirements, and in general with very different performances which may be unpredictable over a long service life. Servo presses, such as presses disclosed in patent application U.S. 60/765,183, sometimes described as having a Direct Drive Chain configuration, do not have a large flywheel and a clutch. A servo motor drives the press directly. At the start of the operation, the motor accelerates the press to a high speed, higher than the pressing speed. Thus the servo motor in a production system including a servo press has the function of driving a cyclic load which changes relatively rapidly with typical cycle times with a duration of some seconds. Then, before impact, the motor slows down the press to pressing speed. Pressing thus occurs at around the same speed as with the mechanical solution. As soon as pressing is completed, the motor once again accelerates the press to high speed. When the press has opened sufficiently for an unloader robot to enter the press, the motor starts slowing down the press. The servo press can thus reach a much improved cycle time at low pressing speeds, because of its capability to run at a high speed during the rest of the cycle.
However, the servo press requires a large motor and power converter (approx. five times larger than the fully mechanical press). Thus for an existing installation, installing a servo motor, such as in a servo press, to a production system may require installing a larger power supply to meet the combined electrical power at the same time as one or more other electric motors are running. In a new installation a larger power supply can be so dimensioned from the start, however, in both cases an increased cost is to be expected.